The toxicity of mercury to humans and the environment has long been known. It is known for example that mercury exposure can cause neurological damage in humans. A particularly devastating example of the harmful effects of mercury occurred in Minamata, Japan in the 1950's where organic mercury byproducts of acetaldehyde production were discharged into the local bay. The byproducts were consumed and metabolized by fish. By consuming fish in the bay, wide spread neurological damage and birth defects among the local population were reported.
Coals used for generating electric power often contain about 0.1 ppm mercury. In the United States alone, about 50 tons of mercury are discharged as vapor in stack gas every year. Through chemical and biological processes, this mercury can become concentrated in fish by many thousand fold, thereby entering human food supplies at harmful levels.
The effort to remove trace mercury from air, water, natural gas, and other industrial streams has a long history, however, removing mercury from coal burning flue gas streams is a very different problem.
Prior art techniques for removing mercury from air or hydrocarbons at room temperature generally have limited relevance to removing mercury from flue gas streams. Mercury has a high atomic weight and adsorption temperature is a significant issue. At room temperature, the dispersion interaction with carbon is sufficient to immobilize mercury atoms. At about 300° F. (the temperature of many flue gas streams), however, physical adsorption is no longer able to hold down the volatile elemental mercury.
In addition, sufficient contact time with rapidly moving flue gas streams is another issue for mercury removal. The total time for flue gas, from generation by combustion to exit through the stack, is often less than 10 seconds. Either as injected powder, where adsorbent fly amid flue gas is for about 2 seconds, or as filter cake on bags in a bag house, the contact time between flue gas and activated carbon captured by the filter is less than one second.
The demand on reactivity and reaction kinetics by flue gas cleaning cannot be properly tested by conventional packed beds. Conventional packed beds are insufficient for flue gas cleaning because the volume of flue gas is so large, the cost for compressing it to push it through a packed bed is prohibitive. For “in-flight adsorption” or “filter adsorption”, the contact time with flue gas is many orders of magnitude less than conventional packed beds. Consequently, test results from conventional packed beds are of limited relevance for flue gas.
Further issues relating to the removal of mercury from flue gas include the small concentration levels of mercury in the flue gas streams. The concentration of mercury in flue gas streams is in μg/m3 whereas the concentration of mercury in many other industrial processes is on the order of mg/m3 (such as garbage incineration). Much early work involved streams other than coal burning boilers and considered effluents containing mercury in the 5 μg/m3 range as fully purified. That is not much lower than the initial concentration of mercury in the flue gas.
Above all, prior art techniques consider the adsorption of mercury as an event between the adsorbent and the mercury. While this is true in air or hydrocarbon streams at room temperature, flue gas contains highly polar and reactive components that can play both an interfering and enabling role for mercury removal. One model composition used for flue gas contains about: 6% O2, 12% CO2, 8% H2O, 1600 ppm SO2, 400 ppm NO, 50 ppm HCl, 20 ppm NO2, and 12 μg/m3 elemental Hg.
Prior art attempts to remove mercury from flue gas of coal burning boilers have included various techniques. One approach has focused on adding halogen salts into coal prior to combustion such that the combustion process generates hydrogen halide gases and then injecting powder carbon downstream into the flue gas at a lower temperature. Some mercury is captured by interaction between the hydrogen halide gases, activated carbon and mercury. Another approach has been to add hydrogen halides or elemental halogen together with activated carbon to a lower temperature flue gas.
U.S. Pat. No. 1,984,164 to Karlsruhe (1934) proposes carbon or silica gel or other adsorbents impregnated with elementary halogen for removal of mercury from room air. Other prior art attempts have included adding halide salts to coal before combustion since these salts are known to be very stable. The combustion process oxidizes halides to halogen and further reacts with hydrogen to yield hydrogen halides. For example, U.S. Pat. No. 5,435,980 to Felsvang et al. (1995) suggest adding chloride or a chlorine containing material into the coal before or during combustion or adding HCl into flue gas upstream of or in the drying-absorption zone.
U.S. Patent Application No. 2004/0003716 A1 to Nelson, Jr. discloses a method for removing mercury and mercury containing compounds from combustion gas by injecting an adsorbent into the flue stream. The sorbent is prepared by treating a carbonaceous substrate with a bromine containing gas. Bromine gas is known to be highly toxic by inhalation, ingestion or skin contact. HBr is also known to be corrosive. In addition, bromine and HBr compounds are reactive and can easily be added onto alkenes. Further, bromine is reactive with aromatics.
U.S. Pat. No. 6,533,842 B1 to Maes et al. (2003) disclose powder adsorbents which contain about 40% carbon, 40% calcium hydroxide, 10% cupric chloride and 10% KI3 impregnated carbon to remove mercury from a high temperature, high moisture gas stream.
In December 2000, the United States Environmental Protection Agency (EPA) made its regulatory decision that mercury emissions from coal-fired electric generating plants need to be controlled.
In the field of the mercury removal from flue gas streams, it would therefore be desirable to provide adsorbents having improved adsorbent characteristics in the flue gas temperature range and that can be economically and efficiently manufactured.